Over 3 trillion pounds of plastic are consumed annually. Since most plastics are based on petrochemical products, the cost of plastic is very dependent on the cost of oil. So, as the cost of oil increases, the cost of the plastic will also increase.
Recently, carbon neutral plastics have become popular. These plastics are based on the use of agricultural feedstocks. The use of agricultural feedstocks does reduce dependence on petroleum; however its cost is impacted by the price of food. Furthermore, plastics produced from agricultural feedstocks are technically carbon-neutral, they still require large amounts of fuel to produce, and are affected by the volatility of global food markets.
Lignin is a natural phenolic type molecule found in wood, straw, corn stover, sugar cane, and virtually all other plants. Lignin is a byproduct of the paper pulping process and cellulosic ethanol production. The waste byproduct of the pulp and paper industry is called black liquor which contains lignin, sugars, organic acids, and inorganic chemicals. Lignin in the black liquor waste stream is primarily burned to recover its heat value and inorganic chemicals. In the United States alone over 50 billion pounds of lignin are burned annually, and over 100 billion lbs are burned throughout the world. Lignin can be used to meet the various applications of the plastics industry without the cost or environmental concerns associated with plastics produced from petroleum or agricultural feedstocks.
To meet the needs of the global plastic industry, lignin must be both extracted from the black liquor waste stream in a cost effective process and the extracted lignin must be compatible with plastic applications. The inventors believe existing lignin extraction methods are not cost effective and the extracted lignin is not compatible with most plastic applications.
The primary method of lignin extraction from black liquor consists of pH reduction until lignin precipitates from the aqueous solution. Black liquor has a pH ranging from 10-14 depending on the pulping process and concentration. Lignin will precipitate from the black liquor solution when the pH is reduced to 1-3. Typically, strong acids such as sulfuric or hydrochloric acid are used to reduce the pH.
The pH reduction causes the extracted lignin to contain significant amounts of inorganic ions. The pulping process and a mineral acid used to reduce the pH contribute to the inorganic ions in lignin. Inorganic ions can partially or completely inhibit reactions necessary to make lignin based plastic. The inorganic ions can be removed using different washing methods. One washing method consists of using a solution of water and an organic acid such as acetic acid. The organic acid solution helps remove inorganic ions from the precipitated lignin, but also increases lignin cost.
The pH reduction fundamentally changes the shape of the lignin molecule. The lignin molecule becomes spherical which causes the functional groups and reactive sites to no longer be accessible. As a result, extracted lignin becomes insoluble in water and many solvents, and is not compatible with many plastic applications.
In addition, pH reduction also causes the lignin molecules to react with each other creating lignin molecules of larger molecular weight. When lignin with large molecular weight is used in plastic applications, then the viscosity will significantly increase. The viscosity increase is undesirable for plastic manufacturing.
Additional processing is needed to compensate for inorganic ions, spherical shape, and larger molecular weight associated with lignin produced by pH reduction. The additional processing requires significant amounts of energy, reagents, and processing. As a result lignin extracted using pH reduction methods is not cost effective and cannot be used in many plastic applications.
The isolated lignin may be dried to remove all water creating an isolated lignin solid. Standard water removal processes are used including but not limited to: vacuum distillation, falling film evaporation, rotary evaporation, reverse osmosis, or oven drying.